Electrochemical impedance spectroscopy and X-ray photoelectron spectroscopy study of the response mechanism of the chalcogenide class membrane iron(III) ion-selective electrode in saline media

The response mechanism of the iron(III) chalcogenide glass membrane ion-sel ective electrode (ISE) in saline media has been studied using electrochemic al impedance spectroscopy (EIS) and X-ray photoelectron spectroscopy (XPS). EIS equivalent circuits and XPS surface compositions for the Fe-III ISE ar e consistent with the presence of two surface films probably comprising a o uter surface layer (OSL) and an Fe-deficient modified surface layer (MSL), along with a low-frequency charge-transfer impedance that is attributable t o the reduction of Fe3+. In accordance with literature data for the conduct ivity of low-bearing iron(III) chalcogenide glasses, a high-impedance MSL i s internally consistent with XPS data for an Fe-deficient MSL. It is eviden t that the impedance of the MSL diminishes on exposure to solutions contain ing Fe3+, and this finding is consistent with the ion exchange of Fe3+ with in the MSL. Likewise, the charge-transfer impedance also decreases at eleva ted levels of Fe3+, demonstrating that Fe3+ is a participant in the reversi ble charge-transfer reaction occurring at the electrolyte/electrode interfa ce. The kinetics of charge transfer are facilitated by Fe chelating agents (e.g., citrate, salicylate, EDTA, etc.) due presumably to the complexation of the products of the charge transfer process (possibly Fe2+). It is shown unequivocally that the response of the Fe-III ISE in saline buffers is ind ependent of pH, demonstrating that the ISE is responding directly to Fe3+, not H+. A mechanism involving a combination of charge transfer and ion exch ange of Fem, at the electrode diffusion layer, has been proposed to explain the 30 mV/decade slope of the Fe-III ISE.